The use of two stage gas furnaces has become quite pervasive recently. Two stage gas furnaces essentially burn gas at two rates, one low and the other is high. In most situations, the low combustion rate is sufficient to satisfy the heat loss from a conditioned environment (e.g., a home). However, when the outdoor environment becomes quite cold the second high combustion rate is used to satisfy the increased demand. The greatest advantage to the two stage furnace is that it allows the furnace to maintain the conditioned environment in a more stable manner. For example, a single stage heating system for a home may be designed to maintain the indoor temperature at 70.degree. F. while the outdoor temperature is 0.degree. F. Depending on the insulation factors and the size of the relevant building, this could require a 100,000 BTU/Hr combustion rate on the furnace. However, if the outdoor temperature is 45.degree. F., the capacity (above the actual need) of this furnace will cause the temperature to exceed a selected desired setting, such as 70.degree. F., in the home (actually caused by the latent heat stored in the heat exchanger of the furnace). The two stage furnace corrects this over-temperature problem by operating at a low fire condition (approximately 50,000 BTU/hr for a 100,000 BTU/hr furnace). This improves the efficiency and comfort level in the home.
In the prior art, two stage gas furnaces required the use of a two stage room thermostat in which the second stage of the furnace is engaged when the second stage of the room thermostat is energized (a separate set of contacts set to close 2.degree. F. below the first stage setting). Another approach is disclosed in U.S. Pat. No. 5,271,556. In this patent, a jumper is placed between the first stage signal (W1) and the second stage signal (W2) to the furnace. The control within the furnace then detects the simultaneity of the two signals when a single stage thermostat is used. The furnace then delays the operation of the second stage of combustion for 5 minutes. This allows the home owner to use a single stage room thermostat with a two stage gas furnace.
The former approach requires a special thermostat while the latter approach complicates wiring and increases the possibility of mistakes being made when the system installer is adding or replacing the jumper.